The invention relates generally to storage systems for liquids and hazardous waste and more particularly to a liquid and waste containment system for short term and long term storage of material which provides extended storage integrity, prompt leak sensing, and leak containment.
The safe storage of materials comprehends the extremes of a time continuum. In the case of hazardous waste such as chemicals, radioactive materials and the like, time may be measured in decades. With regard to chemicals and petroleum products used in industry and commerce, the storage time may be measured in days, weeks or months. Nonetheless, storage of these materials presents many of the same challenges. In both cases, the most serious problem that can occur is the failure of the storage vessel to provide secure storage. The failure may be slow, caused by minute pinholes or cracks, and at first appear as a small flow which gradually builds to a significant flow. Conversely, the failure may be relatively rapid, caused by spontaneous rupture of the vessel, resulting in substantially instantaneous release of its entire contents. In the first instance, the suitable response is typically detection. In the second, it is containment.
Turning to the first area, leak detection, a significant difficulty is the time delay between the onset of a leak and its detection at a location typically remote from the storage container. In large installations, it has been found impractical to provide a leak or flow sensor for every tank or container and thus, typically, the periphery of the storage area is ringed with leak sensors. The time for a leak to travel from a container to a sensor can be significant if the soil is dense or well compacted. Thus, a significant amount of material may escape before a sensor detects and responds to a leak.
In contrast, in smaller installations, a leak detector may be nonexistent or disposed at a significant distance from the container to monitor general underground conditions such as aquifer contaminants. Again, significant time and correspondingly significant amounts of material may escape before a sensor detects a leak.
In either case, a problem compounding the difficulty is often the cleanup associated with such a leak. If large regions of soil have become contaminated by particularly toxic materials, they may need to be removed and stored, further adding to the overall magnitude of the storage problem. It is thus apparent that prompt leak detection is essential to any sophisticated underground tank storage system.
The other above-referenced problem of underground storage is that of containment should the storage vessel rupture. In order to prevent uncontrolled flow of material from a ruptured vessel, it is necessary that a second containment structure fully surround the first vessel. Since it is most probable that a storage tank or vessel will rupture when fully filled, it is necessary that the containment structure define sufficient fluid retaining, i.e., fluid impervious, volume to retain all the material from the largest container. However, since it is unlikely that multiple tanks would rupture, it is generally acknowledged that this minimum volume is also the maximum required volume. If the flow of material from a ruptured tank is contained within a secondary containment structure, difficulties relating to cleanup can be greatly reduced.
Another aspect of underground storage is the life of the storage equipment itself. The longer the nominal design life of the equipment, the safer the system will be and the longer will be the mean time between failures (MTBF). For example, it is known that steel tanks and associated piping have finite life spans ranging from ten years or less in moist soil having a particularly corrosive chemical composition to thirty to forty years or more in dry sand which has no significant chemical effect on the tank. Thus, the overall integrity of the storage system can be improved by minimizing exposure of the primary storage vessel to deleterious ambient conditions.
An alternative to steel tanks are tanks fabricated of fiberglass. Fiberglass tanks include multiple layers of glass fibers, strands or belts secured together by plastic or epoxy resins. Whereas steel tanks are susceptible to deleterious ambient conditions, fiberglass storage tanks are not. The resins, however, are frequently susceptible to attack from ethanol utilized in gasoline/ethanol fuels, i.e., gasohol. It is anticipated that not only the popularity but also the ethanol concentration of such fuels will increase in the future. Since steel tanks are unaffected by ethanol, their increased use in filling stations and similar sites is likely.
The foregoing discussion suggests that improvements in the art of underground storage technology are both desirable and possible.